(1) Field of the Invention
The present invention relates to a developing device for use in a copier or laser printer, for implementing toner supply based on the output data from a toner density sensor so as to keep the density of the toner in a dual-component developer in the developing device constant.
(2) Description of the Prior Art
The image forming apparatus such as a copier, laser printer or the like includes a developing unit arranged near the photoreceptor so as to form a toner image by development with the toner adhering from the developing unit to the electrostatic latent image on the photoreceptor. There are two types of developers to be used for development, namely mono-component type and dual-component type. A mono-component type developer consists of the toner only, so there is no need to control the toner density when the developer is supplied to the developing unit. On the contrary, a dual-component type consists of a toner and a magnetic carrier which electrifies the toner while conveying the toner to the development region where only the toner is consumed for development and the carrier is left in the developing unit. Accordingly, the mixture ratio between the toner and the carrier in the developer varies, so that it is necessary to keep the toner density constant by supplying the toner so as to maintain the quality of image forming.
In a conventional image forming apparatus, a magnetic permeability sensor (toner density sensor) has been used to detect the varying magnetic permeability of the developer with change of the amount of the carrier so as to determine the toner density and thereby control toner supply. Specifically, if the sensor outputs a higher value than the reference level, the toner is added because increase in magnetic permeability is understood to be caused by an increased mixture proportion of the carrier. If the sensor output lowers compared the reference level, toner supply is stopped because decrease in magnetic permeability is understood to be caused by a lower mixture proportion of the carrier.
However, there has been a problem with the toner density sensor that the sensor cannot detect the correct value because the measurement may fluctuate due to humidity and/or the agitated condition of the toner. With regard to humidity, for example, the sensor output is low with a lower humidity while the sensor output is high with a higher humidity for different toner density levels, as shown in FIG. 1. Accordingly, a developer having a toner density of 4% is measured for different humidity conditions, the sensor output increases as the humidity becomes higher as shown in FIG. 2.
This phenomenon can be explained as follows. Under a lower humidity environment, the developer is reduced in moisture content and increased in the amount of electric charge thereon. As a result, repulsion between developer particles becomes stronger so that the developer's volume density decreases, causing the sensor output to be lower. In contrast, under a high humidity environment, the developer is increased in moisture content and reduced in the amount of electric charge thereon. As a result, repulsion between developer particles becomes weaker so that the developer's volume density increases, causing the sensor output to be higher.
Also, as shown in FIG. 3, when the developer has been left unused for a long time, the amount of charge thereon lowers due to discharge, hence repulsion between developer particles becomes weaker so that the developer's volume density increases, causing the sensor output to be higher. When the developer is agitated in the developing unit, the electric charge on the developer increases. As a result, repulsion between developer particles becomes stronger so that the developer's volume density decreases, causing the sensor output to be lower as shown in FIG. 4.
In the above ways, as the developer's volume density decreases with increase in electric charge thereon, the magnetic permeability sensor erroneously detects that the toner density is high. On the other hand, as the developer's volume density increases with reduction in electric charge thereon, the magnetic permeability sensor erroneously detects that the toner density is low.
To deal with this, Japanese Patent Application Laid-open Sho 63 No. 284581 (Patent literature 1) discloses a configuration in which, when due to some variation of the fluidity of the developer, toner oversupply occurs causing foggy images or insufficient toner supply occurs causing lowered image density, the sensor output is manually controlled in accordance with the fluidity so as to shift the toner density output characteristics, thereby providing good images.
Japanese Patent Application Laid-open Hei 4 No. 19765 (Patent literature 2) discloses another configuration in which the output of the magnetic permeability sensor is attempted to be modified by measuring the time that is taken for the supplied toner to travel one circulation of the toner conveyance path formed of two screws with the permeability sensor, determining the fluidity based on the time for circulation, and estimating the volume density from that fluidity.
In the technology of Patent literature 1, however, the shifting means of the sensor output is actuated only when the user notices the occurrence of oversupply or insufficient supply of toner. That is, this configuration is not a one that detects the cause of image degradation and controls the toner density before the occurrence of image degradation.
In the technology of Patent literature 2, though the output from the magnetic permeability sensor is corrected based on the fluidity that is automatically detected, this configuration takes too a long measurement time and hence cannot make real-time correction because the sensor output needs to be corrected by measuring the time that is taken for the toner to travel one circulation of the toner conveyance path by the permeability sensor, determining the fluidity based on that time for the toner making one circulation and estimating the volume density from that fluidity.